Series transformer.



L. W. CHUBB.

SERIES TRANSFORMER.

APPLICATIQN FILED APR. 12. I915.

Paientedsept.3,l9l8i REQ Q WITNESSES:

ATTORNEY @aMM STATES PATENT OFFICE.

LlwIs w. CHUBB, or nnenwoon PARK, PENNSYLVANIA, AssI eNon TO WESTINGHOUSE nnnc'rmc AND MANUFACTURING COMPANY, A conronATIoN or PENNSYLVANIA.

SERIES TRANSFORMER;

Specification of Letters Patent. Patented Sept. .3, 1918,

- Application filed April 12, 1915. Serial No. 20,686.

current-type transformers which supply energy at low voltages to trip coils of circuit breakers or other devices and to electrical measuring instruments.

More particularly, my invention relates to current or series transformers which are adapted to be inserted in power circuits for supplying energyto ammeters, current coils of wattmeters or other devices, etc.

Power circuits that supply energy to electrical apparatusare subject to short circuits which cause abnormal electrical currents to flow over the power mains. Series or current transformers of the usual type, that are inserted in such mains, are liable to injury because of the high mechanical stresses developed, in addition to, the dangerous efiects arising from the abnormally high currents thus induced in the secondary windings. The coils of ammeters, wattmeters, etc., connected in circuitwith the secondary windings of these transformers are frequently burned out because of these high secondary currents, and the needles of such 'instru ments are frequently damaged as a result of I abnormal deflections.

By means of my invention, I provide a series transformer with a core made of a magnetizable alloy which becomes substantially saturated when subjected to low mag.-

netizing forces or magneto-motive forces of low values. It is well known that, in series transformers, the magnetic cores are normally operated at relatively low inductions. However, when abnormally high currents,

v such as short circuitcurrents, flow through the primary windings, the magnetic-cores of the usual series transformers become saturated at high magnetic inductions, there by inducing abnormally high currents in the secondary windings. If these usual iron magnetic'cores are replaced by cores made of a material which becomes fully saturatedby low magnetizing forces,'the cur: rents induced in the secondary windings under abnormal conditions maybe limited to safe values. For example, in transformers in which the magnetizable cores become substantially fully saturated at low magnetic inductions, currents induced in the'secondary windings are limited to safe values irrespective of the value of the short-circuit currents flowing in the primary windings, the cores,

' in this instance, acting in a manner similar to air cores.

In the operation of a current transformer, f

it is desirable that, within the working range, the magnetlzing current be maintained at a minimum value in order to obtain an accurate ratio oftransformation, this result being attained by the use of a core member of high permeability. .Furthermore, it

is desirable that, under overload conditions,"

the secondary current be prevented from attaining excessive values, demanding a core member of low permeability. These two apparently conflicting actions are obtainable by the use of nickel-iron alloys of such a per-cent. composition as to have a magnet izing curve rising steeply from the-origin and suddenly turning to a direction parallel to the axis of abscissae. By so designing the core member that, within the working range, the magnetization is not carried beyond the knee, the desired accuracy in the transformation ratio is secured and, under overload, the secondary voltage does not rise to a harmful degree.

One object of my invention is to provide a series transformer of the above-mentioned character which may be used in conjunction with electrical measuring instruments and the like to protect them against the harmful effects of abnormally high currents or short-circuit currents that may flow in the circuits in which the primaries of such transformers may be connected.

For a better understanding of the nature and scope of my invention, reference may be had to the following description and the accompanying drawings in which Figure 1 is a wow, partially in elevation, and par tially' in section, of a transformer embodying a form of my invention Fig. 2 is a diagrammatic view showing themethod of con necting series transformers in power circuits, and Fig. 3 is a-graph showing several magnetization curves of the materlal from which the magnetizable cores of my trans-' formers are made,

In Fig. 1, the transformer shown comprises .an insulating sleeve or bushing 1 which may be of any suitable structure capable of sustaining very high voltage, annular core members 2 and 3 which surround the bushing 1 and are concentrically located thereon, low-voltage coils l and 5 which are wound'upon the core members 2 and 3, respectively, a supporting frame 6 and a spacing insulator T. The supporting frame 6 consists of a metal box or casing which is disposed upon a pedestal S. The insulating spacing structure 7 is mounted on the upper portion of the casing 6 and comprises a" series of superposed insulating bells 01' caps 9 which are firmly secured together by cement. A winding 10 which constitutes the primary winding of the series transformer, consists of several turns of insulated eonductor and is formed by threading the conductor back and forth through the sleeve or bushing 1 and allowing a suflicient external loop to pass over the insulating support 7. After the primary or high-tension winding 10 is threaded into place, the several turns of which it is composed are taped together and clamped to a metal cap 11 with which the insulator T is provided. as shown in Fig.

'1. Y The primary winding 10 is supplied with terminals 12 and the secondary coils 1 and 5 are supplied with terminals 13.

By referring'to Fig. 2. it willbe seen that the secondary winding of the. series transformer is inductively interlinked with the primary winding 10, the current flowing in the secondary winding normally being proportional to the current flowing in a power main 11. Under ordinary circumstances, the magnetic induction of a common core 15 is substantially low because of the equality of the opposing primary ampere-turns and the secondary ampere-turns of the coils c011- stituting the series transformer. Inasmuch as the core 15 ofmy transformer which corresponds to the core members 2 and 3 of Fig. 1, is made of a special material that becomes substantially fully saturated at a relatively low magnetic induction. as will be hereinafter explained, the currents induced in a secondary winding 16 are limited to safe values, irrespective of the value of the currents flowing in the primary winding 10.

I propose to make the core members 2 and 3 of nickel-iron alloys which contain suf ficient nickel to give to the alloys the magnetic properties desired. By referring to themagnetization curves of Fig. 3. it will .be apparent what nickel iron alloys are suitable for use in my transformer. A magnetization curve 17' represents the relation between magnetic inductions and magnetizing forces of a sample of iron. As explained in my co-pending patent application. Serial No. 172, filed January 2. 1915. and assigned to the Westinghouse Electric and Manufacturing Company, the maximum degree of magnetic permeability at high induction, may be imparted to iron and steel by the addition of amounts of nickel ranging from 1% to 7% of the alloy. Curve 18 is a representation of the magnetization curve of a nickel-iron alloy containing substantially 6% of nickel. Further increases of the percentages of the nickel content in nickel-iron sented by these curves. is low at low magneto-motive forces.

As is well known in the art, nickel-iron alloys containing from substantially 23% to substantially 28% of nickel are non-magnetic. or nearly non-magnetic. and. consequently. their magnetization curves ma be represented. for purposes of illustration. as corresponding. in some degree. to the base line of the X-axis of the graph in Fig. 3. Moreover, the curves 19. 20. 2l and 22 conform. in some degree, to the shape of the curve 18, the samples of these alloys becoming substantially fully saturated at high magnetizing forces only. By fiu'ther increasing the percentages of the-nickel content in the nickel-iron alloys. the magnetization curves rise rapidly at the origin withoutany inflections. as indicated by curves 2, 21 and 25. In these curves, it. will be noted that the samples of nickel-iron alloys become substantially fully-saturated at low magnetic. induct-ions when subjected to relatively low magnetizing forces. Alloys having characteristic curves similar to the magnetization curves 23, 21 and 25 are suitable for cores in my transformers because these curves lack the inflections at the origin that are indicated by the curves 18. 19, 20, 21 and 22. and indicate. that the maximum permeability of the alloys containing nickel contents above 29% occurs at low magnetizing forces. Moreover, the shape of these curves conforms. in some degree and in the regions of relatively high magnetizing forces. to curve 26 which represents the magnetization curve of nickel. Curve 25' represents the magnetization curve of an iron-nickel alloy containing about 35% nickel. This curve represents suflicientlyi well for the that a complete series of these magnetization curves indicates that the magnetic induction of those alloys containing above 29% nickel reaches a maximum value when that those nickel-iron alloys containing at least 29% nickel and, preferably, about 33% to 35% nickel, are highly desirable for use as magnetizable cores of series transformers.

The permeability of these nickel-iron alloys,

' at low magnetizing forces, is substantially constant and reaches a maximum value at very low magnetizing forces. Therefore, such nickel-iron alloys are desirable for use in series transformers Moreover, these alloys become substantially fully saturated at relatively low magnetic induction when subjected to low magnetizing forces, as is inclicated by the maghetization curves 23, 24 and 25. Since cores, constructed of nickeliron alloys containing over 29% nickel and less than nickel, become fully saturated at low magnetic inductions and when subjected to low magneto-motive forces, it will be seen that the currents induced in the secondary winding 16 will be limited to safe values, irrespective of the values of the currents flowing in the primary windings 10 of the series transformer.

Of course, it will be understood that the nickel-iron alloys employed in my series transformer must be heat-treated or annealed 1n a proper manner. The heat treatments which produce alloys that give magnetization curves similar to curves 22, 23, 24 and 25 of Fig. 3, are those between 600 to 1000 centigrade. The claims appended herewith are to be interpreted in accordance with the proper heat treatments being imparted to the alloys. Again, impurities in the alloys may cause a variation of a few per cent. of the nickel content contained in the alloys, but I desire to claim those nickel-iron alloys which have magnetization curves such as are represented by curves-22, 23, 24 and 25 of Fig. 3.

While I have shown and described a particular type oftransformer, it will be apparent to those skilled in the art that any type of transformer may be used in which the core members comprise nickel-iron alloys that contain over 29% nickel. The nickel content may be varied according to the particular uses to which the transformers are to be employed, but I desire to-claim, for use in transformers, all those nickel-iron alloys which contain nickel contents above those contained in the non-magnetic nickel-iron alloys.

"I claim as my invention:

1. A series transformer having a primary coil, a secondary coil, and a magnetizable core interlinking both of said coils which comprises a nickel-iron alloy containing over 29% nickel- 2. A series transformer having a primary winding, a secondary winding, and a common magnetizable core therefor, said core comprising a nickel-iron alloy containing over 29% nickel.

3. A- series transformer having a primary winding, a secondary winding, and a common core therefor, said core comprislng a IllCkBl-IIOH alloy contalnmg nickel 1n eX- cess of that contained in the non-magnetic nickel-iron alloys.

4. A series transformer having a primary winding, a secondary winding, and a common core therefor, said core comprising nickel substantially in excess of that contained in the nickel-iron alloys of low permeability.

51 A series transformer having a pri- -mary winding,a secondary Winding, and a common core therefor, sai core comprising a nickel-iron alloy containing at least 29% nickel. p

6. A series transformer having a primary-winding, a secondary winding, and a common core therefor, said core comprising a nickel-iron alloy containing substantially 33% to 35% nickel. 7. A series transformer having a primary winding, a secondary winding, and a common core therefor, said core comprising a nickel-iron alloy which contains a sufiiciently-high nickel content to become substantially fully saturated at low magnetic induction when subjected to low magnetizing forces. V

In testimony whereof, I have hereunto sullagcribed my name this 31st day of March, 19

EWIS w. CHUBB. 

